1. Field of the Invention
The present invention pertains to a capacitive sensor of a torsion angle and to a torque or moment measuring instrument provided with a sensor such as this. More particularly, the present invention pertains to a sensor such as this and an instrument such as this designed to provide for constant monitoring of a torque transmitted by an engine shaft, of which the sensor is an integral part.
2. Description of the Prior Art
The measurement of the torque transmitted by an engine shaft can be used to determine the mechanical power transmitted by this shaft when its rotational speed is known. Many instruments have been designed to measure torque. These instruments use a very wide variety of sensors such as piezo-electrical sensors, magnetostrictive sensors, inductive sensors, optical sensors, magnetic sensors, wire strain gauge sensors, capacitive sensors etc.
For example, there is a torsional torque or moment detector comprising a tube in which there is a capacitor plate fixed to one of the two ends of the tube. This tube further contains at least one second supporting plate fixed to other one of the two ends of the tube, parallel to the capacitor plate. This supporting plate bears, on its side pointed towards the capacitor plate and on either side of the median line extending parallel to the axis of the torsion tube, two conductive coatings forming capacitor electrodes. The sensor is built into a clamping torque limiter screwdriver. In this application, the torsion tube is fixed to a pneumatic motor which is itself fixed to the casing of the screwdriver. Because of this fixedness, the electric connection of the electrodes to an external device or the measurement of the capacitances of these capacitors raises no particular difficulty and works simply by means of electrical conductors which go through a base of the detector. However, this detector is not adapted for incorporation in a rotating shaft, especially because of its dimensions.
The different prior art measuring sensors prove to be ill-suited to the measurement of torque values developed by engines used, for example, in aeronautics, such as helicopter engines. For, these sensors poorly withstand difficult environmental conditions:
engine shaft located on a housing filled with oil for the cooling and lubrication of the engine; PA1 high temperature and vibration levels; PA1 high rotation speed.
Apart from the difficult environmental conditions, difficulties also arise from the fact that there is limited space to install a torque sensor and that high precision of measurements is required. For, the torque delivered by a helicopter engine is an essential piloting parameter. The monitoring of the development in time of the torque given by the engine gives an indication of its ageing and makes it possible to start maintenance operations and, in the case of two-engine helicopters, it is necessary to balance the torque delivered by the two engines.
At present, for this application, when the measuring instrument uses a toothed wheel type magnetic proximity sensor, the precision of the measurement remains restricted because, firstly, the signals given by the magnetic sensor have little relative variation with the torque transmitted and, secondly, depend on variations of the gap between the toothed wheel and the sensor and the rotation speed of the shaft.
With a torque measuring instrument using a sensor sensitive to the pressure of a hydraulic circuit, the torque of which modulates the section of a leakage hole, the precision is restricted by that of the pressure sensor and by the fabrication quality of the associated hydraulic circuit.
With a measuring instrument using wire strain gauges, there appear the usual drawbacks of strain gauge bridges which deliver low level signals which are difficult to compensate for in terms of temperature.